A wavelength conversion laser for performing, based on the nonlinear optical phenomenon of a wavelength conversion element, wavelength conversion for converting the wavelength of a fundamental laser into converted waves such as a second harmonic (Second Harmonic Generation: SHG), a sum frequency (Sum Frequency Generation: SFG), a difference frequency (Difference Frequency Generation: DFG) and the like has been developed. Above all, since an inner resonator-type wavelength conversion laser in which a wavelength conversion element is inserted into a solid state laser resonator uses a resonator structure, it is characterized in being able to perform highly efficient wavelength conversion.
In addition, among the solid state lasers, a micro chip solid state laser using a laser medium of sub mm to several mm is particularly compact and able to obtain a W-class output, and various applications thereof are anticipated. Application of the combination of a micro chip solid state laser and an inner resonator-type wavelength conversion laser is being attempted in the wavelength range that cannot be directly oscillated with a semiconductor laser, and in ranges that are demanded of giant pulses or high frequencies.
Conventionally, proposals have been made for using a plurality of crystals as the solid laser medium in order to improve the characteristics of the solid state laser and the wavelength conversion. Patent Document 1 attempts to achieve the high output of laser by arranging a plurality of crystals with a different rare earth ion concentration but of the same compositional formula as the laser medium in ascending order of concentration. Moreover, Patent Document 2 proposes a laser comprising a plurality of laser media including a fluorescence spectral range in which at least a part thereof mutually overlaps in the resonator, wherein the fluorescence spectral width is enlarged to cover a broad wavelength range. In addition, Patent Document 3 proposes a configuration of using two types of solid laser media and three reflecting mirrors to configure a resonator of two wavelengths that share one reflecting mirror, and performing sum frequency mixing based on nonlinear optical crystals.
Nevertheless, although the conventional configurations proposed above are able to improve the respective characteristics, they do not give consideration to the fact that there will be substantial fluctuations in the output and characteristics when the laser is subject to a temperature change.
Furthermore, with respect to the application to fields of visual media, lighting and the like, the inner resonator-type wavelength conversion laser has problems in that the spectrum width is narrow and interference noise in the form of a splash known as speckle noise is generated. Specifically, with the inner resonator-type wavelength conversion laser, the achievement of a single mode and narrow band of the solid state laser were required in order to enable highly efficient wavelength conversion. However, it is necessary to broaden the spectrum width of the wavelength conversion laser beam in order to reduce the speckle noise. In addition, even in cases where a temperature change or the like occurs, a stable and broad spectrum width of the wavelength conversion laser beam is being sought.
Patent Document 1: Japanese Patent Application Laid-open No. 2005-93624
Patent Document 2: Japanese Patent Application Laid-open No. 2008-4882
Patent Document 3: Japanese Patent Application Laid-open No. 2006-66436